Tetanus - Horse

Also known as:

Lockjaw

Description

Tetanus is a highly-fatal infectious disease caused by the neurotoxin of the gram-positive bacterium Clostridium tetani, resulting in spastic paralysis of affected animals. Clostridium tetani causes infection via wound contamination and deposition of bacterial spores in tissue under anaerobic conditions. Cuts, deep puncture wounds of the foot and surgical sites may all provide sites of entry for infective spores. C. tetani produces tetanospasmin, a neurotoxin that is transferred to distant sites via the vascular system. This causes inhibition of the release of GABA and glycine from interneurons in the spinal cord and brain, resulting in tetanic spasms. The incubation period varies from one to four weeks.

Signalment

All domestic species of all ages may be affected but foals may be particularly susceptible due to the possibility of infection through the umbilicus. Mare with a retained placenta post-foaling may also be affected.

Diagnosis

Diagnosis is often based on history of a penetrating wound and clinical signs. Affected animals may have no history of vaccination against tetanus. Severe cases may be easily diagnosed but mild cases may need differentiating from exertional rhabdomyolysis, laminitis, hypocalcaemia and heat stroke.

Clinical Signs

• Sudden onset stiff gait
• Rigid posture ('sawhorse stance')
• Protrusion of the third eyelid, particularly after mild stimulation
• Trismus (lockjaw)
• Nostril flaring
• Retracted lips
• Elevated tail head
• Dysphagia
• Tetanic spasms of the neck, trunk, masseter and limb muscles

Cases may vary from mild signs of a short, stilted gait, to complete recumbency with spastic paralysis. Progression of the disease may lead to involvement of the pharyngeal and laryngeal muscles leading to aspiration pneumonia. Recumbency may be followed by paralysis of the respiratory muscles, leading to death due to hypoxia or respiratory arrest.

Treatment

Treatment may be attempted but consideration should be given before embarking on treatment of severely affected cases, as these carry a grave prognosis (e.g. recumbency, severe muscle spasms). Horses should receive as little external stimuli as possible, ideally by keeping them in a darkened, quiet stable. Administration of acetylpromazine may provide both sedation and a reduction in muscle spasm. The use of an intravenous catheter is recommended to further reduce external stimulation. Any obvious wounds should be thoroughly debrided and flushed using 1% hydrogen peroxide, and parenteral penicillin should be administered. Tetanus antitoxin may be adminstered intravenously at the onset of clinical signs.

• • Surgical removal of dead and infected tissue from the wound.
• Antibiotic treatment.
  • Metronidazole
    • Treatment with metronidazole decreases the number of bacteria but has no effect on the bacterial toxin.
    • Metronidazole is particularly active against anaerobic bacteria, and so is a good choice for use against C. tetani
  • Penicillin
    • Penicillin is becoming a less favoured treatment for tetanus, since there is a theoretical risk of increased spasms.
• Muscle relaxants
  • Diazepam
  • Midazolam
• Supportive therapy, for example:
  • Administering fluids
  • Providing appropriate nutrition
  • Maintaining a quiet environment with dimmed light.
• Remission take several weeks to months

References

• Blood, D.C. and Studdert, V. P. (1999) Saunders Comprehensive Veterinary Dictionary (2nd Edition) Elsevier Science

• Mair, T., Love, S., Schumacher, J. and Watson, E. (1998) Equine Medicine, Surgery and Reproduction WB Saunders Company Ltd